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sodium are only required in appreciable amounts by organisms normally living in salt liquids, i.e., marine and many pathogenic organisms. Like all other organisms, the microorganisms are able to derive the necessary sulphur, phosphorus and metallic elements from inorganic salts. These salts are found in the required proportions in plant ashes, and formerly it was the practice to add them to nutrient media in this form. If microorganisms are cultivated in milk or the extracts of meat or plants they will generally be well supplied with the requisite inorganic nutriment. Nitrogen. Microorganisms may be divided into two main groups according to their ability or inability to assimilate all the nitrogen they require from inorganic sources. A few species belonging to the former group can assimilate nitrogen from the air, and thus incidentally improve the soil for plant growth. The great majority of these, however, require their nitrogen in the form of ammonia or nitrates ; they are represented by the typical water bacteria, acetic acid bacteria, and many yeasts and moulds. Belonging to the group which cannot grow in the absence of proteins or the immediate decomposition products of proteins are many of the putrefactive bacteria and the true lactic acid bacteria. Carbon. Microorganisms fall into two groups with respect to 28 DAIRY BACTERIOLOGY later, according to Migula, motile rods, while now it generally denotes spore-forming rods. The screw-shaped bacteria, which always have polar flagellse, are classified into the genera Vibrio and Spirillum. The vibrios are monotrich and form single curves, being commashaped ; the spirilla are lofotrich and more or less screwshaped. Certain spirilla are said to be able to form spores. Fiu. 30. Vibrio cholerce.